Preheat the oven according to the directions on the box.
Line the muffin tin with cupcake liners or silicone liners.
In a large mixing bowl, prepare the batter according to the directions on the box and add in the Watermelon flavoring oil.
Using a tablespoon, spoon out 3 tbsp of batter and add it into each cupcake liner.
Place the muffin pan into the oven for 15 - 20 minutes (or according to the directions on the box). Start checking the cupcakes at the 15 minute mark by sticking a toothpick or cake tester into the center of the cupcake.
Remove the pan from the oven and let the cupcakes set in the tin for 10 minutes. The remove the cupcakes from the pan and let them finish cooling on a wire rack.
While they are cooling, make the frosting.
Puree the strawberries in a blender.
Then in a large bowl, add in the powder sugar, vanilla, butter, and the strawberries. Start the mixer on low and mix all of the ingredients together.
Your goal when mixing this together is a thick yet pipeable frosting. Add more powdered sugar as needed.
Once you get the desired consistency, add the the tip into the piping bag and push it into the bottom of the bag. Then trim off the bottom of the bag so the tip comes through. Then add the frosting into the piping bag. Pipe the frosting onto the cooled cupcake and add the sprinkles.